1. Field of the Invention
The present invention relates to a semiconductor device having a configuration in which data stored in memory cells is read out to bit lines and amplified by sense amplifiers.
2. Description of Related Art
Conventionally, semiconductor memory devices each provided with a sense amplifier which senses and amplifies data read out from a memory cell to a bit line have been known (Refer to, for example, Patent References 1 and 2). Further, a memory circuit using a charge transfer amplifier is known in relation to the Patent Reference 1 (Refer to Non-Patent References 1, 2 and 3). Particularly, the Patent Reference 1 discloses a memory cell MC of a gain cell type (a kind of DRAM cells) in which a gate of an output transistor M1 is used as a storage node SN shown in FIG. 8 and description thereof. This memory cell MC stores data in a capacitor (not shown) connected to the storage node SN. Operation of the memory cell MC of the gain cell type is performed so that data is written as a voltage at the gate (storage node SN) of the output transistor M1 and the data is read out by boosting the voltage of the storage node SN via the capacitor. The data is transmitted to a sense amplifier SA from the memory cell MC. Regarding this capacitor, the Patent Reference 1 does not disclose a specific structure or a write method for the capacitor.    [Patent Reference 1] Japanese Patent Application Laid-open No. 2007-73121    [Patent Reference 2] Japanese Patent Application Laid-open No. H11-16384    [Non-Patent Reference 1] L. G. Heller et al., “High sensitivity charge-transfer sense amplifier,” IEEEJ. Solid-State Circuits, vol. SC-11, pp. 596-601, October 1976    [Non-Patent Reference 2] T. Kawahara et al., “Bit-line clamped sensing multiplex and accurate high-voltage generator for 0.25 μm flash memories,” in 1996 ISSCC Dig. Tech. Papers, February 1996, pp. 38-39.    [Non-Patent Reference 3] T. Kawahara et al., “Bit-line clamped sensing multiplex and accurate high-voltage generator for quarter-micron flash memories,” IEEE J. Solid-State Circuits, vol. 31, no. 11, pp. 1590-1599, November 1996
However, the configuration of the above conventional semiconductor memory device is mainly intended to achieve high-speed operation of the sense amplifier, and it is not taken into consideration that a write voltage for the memory cell is set sufficiently lower than an operation voltage of the sense amplifier. If a single-ended sense amplifier using a single MOS transistor connected to a single bit line is employed in order to reduce circuit scale of the sense amplifier, it is particularly desirable to reduce the write voltage. That is, when the write voltage for the memory cell is relatively large, consumption current increases due to charge/discharge current of the bit line. Further, data retention time of the memory cell is shortened due to influence of leak current from the memory cell (data storage node, exactly) to a substrate, and this causes various problems such as an increase in a boost voltage required for driving word lines.
Meanwhile, one of proposed configurations of the sense amplifier used in the DRAM includes a dynamic latch circuit having a plurality of MOS transistors arranged in a cross pattern (in which inputs and outputs of CMOS inverters are cross-coupled to each other). In this kind of the sense amplifier, since a pair of bit lines connected to the sense amplifier is precharged to an intermediate voltage between the write voltage for the memory cell and a ground potential, an operation voltage of the dynamic latch circuit becomes half of the write voltage when starting the sensing operation. In order to reduce the write voltage in such a configuration, if the write voltage is set to, for example, 0.5V, the operation voltage of the dynamic latch circuit becomes 0.25V so as to be approximately equal to a threshold voltage of the MOS transistor. Thus, there arises a problem that it is difficult for the sense amplifier to normally operate, or a problem that operating speed of the sense amplifier increases even if it operates.